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The present invention relates to therapeutic radiation sources, and more particularly to a reduced power, increased efficiency miniaturized radiation source that utilizes an optically driven thermionic cathode.
In the field of medicine, therapeutic radiation such as x-ray radiation and xcex3-ray radiation is used for diagnostic, therapeutic and palliative treatment of patients. The conventional medical radiation sources used for these treatments include large, fixed position machines as well as small, transportable radiation generating probes. The current state-of-the-art treatment systems utilize computers to generate complex treatment plans.
Conventional radiation systems used for medical treatment utilize a high power remote radiation source, and direct a beam of radiation at a target area, such as a tumor inside the body of a patient. This type of treatment is referred to as teletherapy because the radiation source is located a predefined distance from the target. This treatment suffers from the disadvantage that tissue disposed between the radiation source and the target is exposed to radiation. Teletherapy radiation sources, which apply radiation to target regions internal to a patient from a source external to the target regions, often cause significant damage not only to the target region or tissue, but also to all surrounding tissue between the entry site, the target region, and the exit site.
Brachytherapy, on the other hand, is a form of treatment in which the source of radiation is located close to or in some cases within the area receiving treatment. Brachytherapy, a word derived from the ancient Greek word for close (xe2x80x9cbrachyxe2x80x9d), offers a significant advantage over teletherapy, because the radiation is applied primarily to treat only a predefined tissue volume, without significantly affecting the tissue adjacent to the treated volume. The term brachytherapy is commonly used to describe the use of radioactive xe2x80x9cseeds,xe2x80x9d i.e. encapsulated radioactive isotopes, which can be placed directly within or adjacent the target tissue to be treated. Handling and disposal of such radioisotopes, however, may impose considerable hazards to both the handling personnel and the environment.
The term xe2x80x9cx-ray brachytherapyxe2x80x9d is defined for purposes of this application as x-ray radiation treatment in which the x-ray source is located close to or within the area receiving treatment. An x-ray brachytherapy system, which utilizes a miniaturized low power radiation source that can be inserted into, and activated from within, a patient""s body, is disclosed in U.S. Pat. No. 5,153,900 issued to Nomikos et al., U.S. Pat. No. 5,369,679 to Sliski et al., and U.S. Pat. No. 5,422,926 to Smith et al., all owned by the assignee of the present application, all of which are hereby incorporated by reference.
The x-ray brachytherapy system disclosed in the above-referenced patents includes a miniaturized, insertable probe which is capable of generating x-ray radiation local to the target tissue, so that radiation need not pass through the patient""s skin, bone, or other tissue prior to reaching the target tissue. The insertable probe emits low power x-rays from a nominal xe2x80x9cpointxe2x80x9d source located within or adjacent to the desired region to be affected. In x-ray brachytherapy, therefore, x-rays can be applied to treat a predefined tissue volume without significantly affecting the tissue adjacent to the treated volume. Also, x-rays may be produced in predefined dose geometries disposed about a predetermined location. X-ray brachytherapy offers the advantages of brachytherapy, while avoiding the use and handling of radioisotopes. Also, x-ray brachytherapy allows the operator to control over time the dosage of the delivered x-ray radiation.
X-ray brachytherapy typically involves positioning the insertable probe into or adjacent to the tumor, or into the site where the tumor or a portion of the tumor was removed, to treat the tissue adjacent the site with a local boost of radiation. X-ray probes of the type generally disclosed in U.S. Pat. No. 5,153,900 include a housing, and a hollow, tubular probe or catheter extending from the housing along an axis and having an x-ray emitting target at its distal end. The probe may enclose an electron source, such as a thermionic cathode. In another form of an x-ray brachytherapy device, as disclosed in U.S. Pat. No. 5,428,658, an x-ray probe may include a flexible probe, such as a flexible fiber optic cable enclosed within a metallic sheath. In such a flexible probe, the electron source may be a photocathode. In a photocathode configuration, a photoemissive substance is irradiated by a LED or a laser source, causing the generation of free electrons. Typically, the flexible fiber optic cable couples light from a laser source or a LED to the photocathode.
It is possible to reduce the power requirements of miniaturized therapeutic radiation sources used in x-ray brachytherapy, by optically driving the thermionic cathodes in the electron sources, instead of ohmically heating the thermionic cathodes. U.S. patent application Ser. No. 09/884,561 (identified by Attorney Docket Nos. PHLL-155 and hereby incorporated by reference)(hereinafter the xe2x80x9cPHLL-155xe2x80x9d application) discloses a miniaturized therapeutic radiation source that includes a reduced-power, increased efficiency electron source that is optically driven. The PHLL-155 application discloses an electron source that includes a thermionic cathode having an electron emissive surface. The PHLL-155 application discloses using laser energy to heat the electron emissive surface of the thermionic cathode, instead of heating the electron emissive surface of the thermionic emitter using conventional ohmic heating. In this way, electrons can be produced in a quantity sufficient to produce the electron current necessary for generating therapeutic radiation at the target, while significantly reducing the power requirements for the therapeutic devices. Electrons can be generated with minimal heat loss, without requiring a vacuum-fabricated photocathode. By using optical heating, the mechanical complexity of the cathode is greatly reduced.
It is desirable that the surfaces of the thermionic cathodes be heated to as high a temperature as possible as quickly as possible, i.e. that the surfaces be heated as efficiently as possible. In order to reduce the power requirements for the miniature radiation source as disclosed in the PHLL-155 application, it is therefore necessary to minimize heat loss by the thermionic cathode. Heat loss by laser-heated thermionic cathodes generally includes 1) heat lost by thermal conduction; 2) heat loss caused by the portion of incident laser radiation that remains unabsorbed; and 3) heat loss by thermal radiation.
It is an object of this invention to increase the efficiency of a therapeutic radiation source having an optically driven thermionic cathode, by reducing the proportion of incident laser radiation that remains unabsorbed by the cathode. It is another object of this invention to increase the efficiency in the laser heating of a thermionic cathode in a miniaturized, laser-driven radiation source, by modifying the geometry and configuration of the cathode.
The present invention is directed to an optically driven therapeutic radiation source having a laser-heated thermionic cathode. In the present invention, the geometry and configuration of the thermionic cathode are designed to substantially increase the coupling efficiency of the incident laser radiation onto the cathode. This increase in coupling efficiency is achieved by substantially reducing the portion of incident laser radiation that remains unabsorbed by the cathode.
The present invention features a therapeutic radiation source that includes a radiation generator assembly, a source of optical radiation, and an optical delivery structure such as a fiber optic cable. The radiation generator assembly includes an electron source for emitting electrons to generate an electron beam along a beam path, and a target positioned in the beam path and adapted to emit therapeutic radiation in response to incident accelerated electrons from the electron beam. The electron source includes a thermionic cathode having an electron emissive surface. The fiber optic cable is adapted to transmit optical radiation, incident on an proximal end of the cable, to a distal end of the cable. The fiber optic cable directs a beam of the transmitted optical radiation upon the electron emissive surface of the cathode. The beam has a power level sufficient to heat at least a portion of the surface to an electron emitting temperature, so as to cause thermionic emission of the electrons from the surface.
The present invention features a non-planar configuration for the laser-heated thermionic cathode, in contrast to prior art themionic cathodes which have a planar, disk-shaped configuration. For example, the thermionic cathode may have a substantially conical shape, or a substantially convex shape, or a substantially hemispherical shape.
The thermionic cathode is shaped and designed so as to allow an incident beam of optical radiation to impinge upon, and undergo absorption from, a plurality of non-overlapping regions within the surface of the cathode, consecutively in succession. Because the incident optical radiation undergoes absorption processes from a plurality of regions within the surface, the amount of incident laser radiation that becomes absorbed by the non-planar thermionic cathode is substantially increased, as compared to the amount of incident radiation that is absorbed from only one region within a conventional planar, disk-shaped thermionic cathode. In other words, the coupling efficiency of the incident optical radiation to the thermionic cathode is substantially increased by modifying the shape and configuration of the cathode.